1. Field of the Invention
The present invention relates generally to position detectors and, more particularly, to a position detector that comprises two magnetically sensitive devices which combine to provide quadrature sensing.
2. Description of the Prior Art
Many different types of position sensors are known to those skilled in the art, Some position sensors are intended to detect the movement of a target along a linear path while others detect the rotation of a target, such as a gear with a plurality of teeth, about an axis of rotation. The target and sensor can be arranged so that the target is provided with a plurality of magnetic poles that are sensed by a magnetically sensitive component. Alternatively, the sensor may be provided with a biasing magnet and the target can comprise a plurality of ferromagnetic discontinuities, such as gear teeth, that are sensed by the device.
It is also known to those skilled in the art to utilize two magnetically sensitive devices together for the purpose of determining the position and movement of a target. U.S. Pat. No. 5,041,785, which issued to Bogaerts et al on Aug. 20, 1991, discloses a device for measuring a relative displacement of two objects, including a magnetic scale and two mutually perpendicular magnetic sensors which produce two independent phase displaced signals. The device comprises an elongate scale consisting of a succession of magnetic north poles and south poles. It also comprises a measuring head with at least a first and a second magnetic field sensor that are subjected to the effect of a magnetic field which alternates as a function of time due to the alternating passage of north poles and south poles during a displacement of the scale in its longitudinal direction with respect to the measuring head.
U.S. Pat. No. 5,019,776, which issued to Kawamata et al on May 28, 1991, describes a magnetic position detection apparatus that magnetically detects a positional relation between first and second members. It comprises a magnetic recording medium carried on the first member at a magnetic sensor substrate supported by the second member. Two or more tracks each including a multiplicity of magnetic poles arranged at a predetermined pitch are provided on the magnetic recording medium.
U.S. Pat. No. 4,897,914, which issued to Loubier on Feb. 6, 1990, discloses a method of making a magnetic encoding device having Hall effect devices. The method achieves reproducibility by forming a frame of the encoder as a unitary, or one piece, arrangement for both supporting a shaft bearing and an angular behavior sensor such as a Hall effect switch. It therefore controls tolerances, air gaps and angular orientation of the sensor. Calibration of such a sensor to compensate for manufacturing variability in the Hall sensing device as well as any other variabilities in the encoder is then achieved by providing a reversible calibrating magnetic field close to the Hall effect switch and then moving the magnetic field until the desired switch calibration is achieved.
U.S. Pat. No. 4,810,967, which issued to Yokoyama et al on Mar. 7, 1989, describes a position sensor for detecting angular position. It includes a light, compact angle-of-rotation sensor which comprises a magnetic ring bonded to a shaft by means of resin. The magnetic ring is provided around its periphery with a number of magnetized zones, with each magnetized zone having a different number of magnetic poles. Magnetic interference between magnetized zones is prevented by leaving nonmagnetized spaces between the magnetized zone. The prevention of magnetic interference can be further enhanced by the provision of nonmagnetized portions between the magnetized poles of the magnetized zones.
In certain applications where devices of this type are intended to detect the rotation of a rotatable target, it is necessary to detect both the rate of rotation of the target and the direction of rotation. If the target suddenly reverses direction, the sensor must be able to detect that reversal. This type of sensor can find utilization in automatic braking systems and other systems wherein both speed and direction of rotation of a target must be determined.
Quadrature sensing is well known to those skilled in the art. It involves the use of two signals which are offset from each other by 90 degrees so that a comparison of the signals will provide meaningful information with regard to the position of a target. U.S. Pat. No. 5,041,785, described above, utilizes two magnetoresistive elements in association with each other to provide signals of this type.